While weight-management interventions are effective in attenuating antipsychotic-induced weight, there is no available evidence on their long-term effectiveness. This study sought to investigate the 2-year effects of an early behavioural intervention (EBI) designed to prevent antipsychotic-induced weight gain in first-episode psychosis (FEP) patients. Sixty-one FEP patients were randomized to receive either EBI or treatment-as-usual. Intention-to-treat and observed-cases analysis showed that patients in the EBI group gained significantly less weight than those allocated to routine care at intervention completion (3-month follow-up) with treatment effects maintained over 3months. Differences between groups were no longer significant by 12months. Weight-management interventions may need to be offered for longer periods to maintain preventative effects. Alternatively, booster sessions may need to be regularly delivered after intervention completion.